1. Field of the Invention
The invention relates in general to hoist ring assemblies and, in particular, to a high load capacity hoist ring assembly capable of being installed at a required tensile value to an object to be lifted without the use of either a torque multiplier or a hydraulic tensioning device.
2. Description of the Prior Art
Various high load hoist ring assemblies have been proposed previously. For Example, in Tsui et al U.S. Pat. No. 5,848,815, a safety hoist ring was proposed for lifting large loads. However, for a given hoist load capacity rating, the mount stud or shank member of the hoist ring assembly must be torqued to a predetermined value in order to pre-stress that member to the required tensile value. In order to properly pre-stress the shank member, the required tensile value should at least be equal the load capacity rating of the assembly, however, it is preferable that it be at least 1.5 times greater to assure against misuse, such as overloading the assembly, and the like. As the load capacity of the hoist ring assembly is increased, so to is the torque value required to achieve the proper tensile value in the shank member. For instance, a hoist ring assembly of the configuration disclosed in Tsui et al U.S. Pat. No. 5,848,815 having a load capacity rating of 50,000 pounds (lbs.), weighs about 87.5 lbs, and requires a torque value of 2,100 ft-lbs in order to achieve the required tensile value of 75,000 lbs in a shank member having 21/2"-4 UNC threads. When the assembly is increased to a load capacity rating of 100,000 lbs, the assembly weighs 240 lbs, and requires a torque value of 6,800 ft-lbs in order to achieve a required tensile value of 150,000 lbs in a shank member having 31/2"-4 UNC threads. These extreme torque values are impossible to achieve manually with conventional torque wrenches. Expensive torque multipliers or hydraulic tensioning devices are required. For instance, to achieve the torque value of 6,800 ft-lbs in the example above, a conventional torque wrench having a 3 foot moment arm would require the application of 2,266 pounds of pulling force to the wrench by the operator. Thus, as those skilled in the art recognize, the only practical method known to achieve these torque values requires the use of a torque multiplier in combination with a conventional torque wrench, or the use of a hydraulic tensioning device. Undesirably, such devices are expensive, often being twenty times as much, or more, than the cost of the hoist ring assembly itself. In addition, these devices are both heavy and bulky and thereby reduce the range of possible mounting locations for the hoist ring assembly.
It has been found that these problems arise when the desired torque requirements reach about 230 ft-lbs and above. This nominal torque value, 230 ft-lbs, is required when load capacity ratings of the hoist ring assemblies reach around 10,000 lbs.
Those concerned with these problems recognize the need for an improved high load capacity hoist ring assembly capable of being installed to the surface of an object to be lifted at its required tensile value without the use of a torque multiplier or hydraulic tensioning device.
These and other difficulties of the prior art have been overcome according to the present invention.